Add nullable columns without a default

Deploying knowledge bases showed the migration runner doing the wrong thing:

    ALTER TABLE "documents" ADD COLUMN "base_id" VARCHAR(32) DEFAULT ''

`base_id` is nullable and its absent value is NULL, but the runner derived a
default from the column type and backfilled every existing row with the empty
string. Nothing then matched `base_id IS NULL`, so the startup sweep that files
pre-bases documents into a default base would have skipped all of them and the
documents would have stayed invisible.

Nobody lost anything -- the live instance had no documents yet -- but the fault
is general: any nullable column added from here would arrive as "" rather than
NULL, and every "is this set?" check would be wrong about the rows that predate
it. So a default is now emitted only for NOT NULL columns, where SQLite requires
one.

The sweep also accepts "" as meaning unfiled, since a deployment that upgraded
through the previous release has rows holding it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_settings.py

@@ -200,3 +200,34 @@ def test_other_sessions_are_revoked_but_this_one_survives(client: TestClient, db
     assert other.get("/chat", follow_redirects=False).status_code == 303
     # ...and the tab that made the change is still in.
     assert client.get("/chat", follow_redirects=False).status_code == 200
+
+
+# --- Adding a column to a table that already has rows -------------------------
+def test_a_nullable_column_is_added_without_a_default(db):
+    """Backfilling one would give existing rows a value the model does not treat
+    as absent: an added foreign key arrives as "" rather than NULL, and every
+    "is this set?" check downstream is then wrong about the old rows."""
+    from sqlalchemy import Column, String
+
+    from lembas.db.migrations import _add_column_sql
+    from lembas.db.models import Document
+
+    sql = _add_column_sql(
+        Document.__table__, Column("later", String(32), nullable=True), db.bind.dialect
+    )
+    assert "DEFAULT" not in sql
+
+
+def test_a_not_null_column_still_gets_one(db):
+    """SQLite refuses NOT NULL with no default, so there it is unavoidable."""
+    from sqlalchemy import Column, String
+
+    from lembas.db.migrations import _add_column_sql
+    from lembas.db.models import Document
+
+    sql = _add_column_sql(
+        Document.__table__,
+        Column("later", String(32), nullable=False, default=""),
+        db.bind.dialect,
+    )
+    assert "NOT NULL" in sql and "DEFAULT" in sql